Precision liquid dispensing pumps are used in many environments, in which precisely measured quantities of a liquid are required to be dispensed. Examples of such applications are found in the food, cosmetics, pharmaceutical, chemical and battery industry.
A linear liquid dispensing pump is described in U.S. Pat. No. 5,312,233. Here the piston of the pump is reciprocating, rotating. Rotation is effected through a flexible coupling means connected to the rotor of a step motor. The coupling means allows slight alignment differences between the axis of the rotor and of the piston. A different example for an apparatus with servo-motor driven pumps is described in U.S. Patent Publication 2006/0203609. In both of these prior art designs the servo-motor driven pump and a dispensing unit operated by the reciprocal movement of the pump constitute a single mechanical unit. The size of the apparatus is increased by the fact that the same reciprocating actuator can be adapted to several cylinder-plunger pump units, which are all integrated in the combined pump and dispensing units. This property renders cleaning long and difficult and it unnecessarily affects the linear actuator within the apparatus. Furthermore, the design has an increased length and volume and it is difficult to handle.
A different approach is applied in a prior art metering pump with servo-motor driven actuator which is the Model 2S available from Hibar Systems Limited of Richmond Hill, Canada. This system is schematically illustrated in FIGS. 1 and 2. The main part is a servo-motor driven linear actuator 20 which has the task of providing a very accurate linear displacement of an actuator element with a cylindrical hollow frontal bore. Apart from its precisely controlled guided linear axial movement, the actuator element is not rotating or moving in any other direction. The front portion of the actuator assembly 20 has a releasable adaptor socket to which a separate dispensing unit 10 can be attached. The dispensing unit 10, when coupled to the socket of the main actuator assembly 20 has a plunger which is inserted in the cylindrical frontal bore of the actuator so that it follows the precise reciprocating linear motion thereof. The plunger of the dispensing unit 10 co-operates with a cylinder and in each period of the movement it meters a precisely adjusted volume of a liquid inserted in the cylinder. The releasable connection between the main actuator assembly 20 and the dispensing unit 10 makes it possible that a great variety of dispensing units designed for the metering of liquids of differing viscosities and/or volumes can be connected to the same actuator unit, and the user has the freedom of choosing the dispensing unit that fits most to his actual dispensing task.
The problem with this separate servo motor drive assembly lies mainly in its comparatively large size, and for its perfect operation respective separate voluminous power supply unit and a control unit has to be used, which are all wired together to retain functionality. Furthermore, in this known actuator the servo motor rotates the shaft of a ball screw unit, which has a cooperating ball screw nut guided for linear movement in the actuator element, therefore the nut cannot rotate.
In case of servo motor driven linear actuators the most widely-spread solution is the use of customary built servo-motors. The rotation movement is generally converted into a linear movement by means of a ball screw system comprising a shaft and a nut, wherein a separate coupling element is used that connects the rotor of the motor with the rotating shaft of the ball screw unit. Here the length of the rotor, of the coupling element and that of the ball screw are added, which result in rather long geometric dimensions, especially length.
There are several known servo-motor drives that can be used as linear actuators. One example which does not use a separate coupling element between the rotor and the ball screw is described in US 2009/0056485 A1 of Kobayashi et al., wherein the rotor of the servo motor is mechanically connected to an axial ball screw shaft so that sharp steps are made at the rear extension of the ball screw shaft, and the rotor has a conformingly stepped hollow interior for receiving the end of the ball screw shaft. The shaft is attached to the rearmost portion of the rotor. The front portion of the rotor has a larger diameter hollow opening so that a non-rotating ball screw nut travels in and out of this opening, and this linearly moving nut constitutes the actuator element of the drive. A problem lies in the very thin and long design of the ball screw shaft and of the conforming hollow design of the rotor, which provides a degree of instability and a feature of difficult mass-balancing. A further problem lies in the weight of the rotating ball screw which, when attached to the rotor, leads to an effective increase in the weight of the rotating components and therefore correspondingly higher rotor inertias. Since satisfactory operation of the dispensing pump requires rapid acceleration, deceleration and changes in rotation direction of the servomotor rotor, the higher rotor inertia becomes a critical speed limiting factor. A further problem is connected with the travel of a large non-rotating nut in the hollow opening of the rotor, where the guiding is difficult to solve. The servo motor used is of a conventional type, which has standard bearing design for rotational movement and such bearings are less capable of enduring axial forces that are inevitably present in linear actuators. In that design the axial distance between the front and rear bearings of the rotor is short, and the rear bearing has a decreased diameter owing to the narrow design of the rotor end portion. By such constructional limitations the axial forces and any axial misalignment can increase the load acting on the bearings decreasing thereby their life time and the accuracy of the control.
A similar design is shown in U.S. Pat. No. 8,020,462 B2 issued also to Kobayashi et al., wherein the difference compared to the previous design lies in that the ball screw shaft can be connected in the hollow cavity of the rearmost portion of the rotor in a releasable way.
A cylinder servomotor is described in EP 1 182 765 A1, which is a self-contained unit which has a linearly reciprocating shaft that constitutes the output element. In this document the servomotor is built integrally with a ball screw drive, wherein the nut is fixed in the rotor and the threaded shaft is guided for linear movement. The servo motor has a front portion that comprises a radial bore in which two balls are spring-biased to a linear groove tooled in the threaded front portion of the ball screw shaft. The balls are pressed against the groove with V-shaped cross section, and the balls have the task of preventing twisting of the shaft. The threads on the shaft intersect with the groove, and the associated guiding problems are solved by choosing an appropriate size of and distance between the balls. While such a system works fine in case of comparatively small axial loads, the twisting forces acting on the guiding element increases with load. In case of higher twisting forces, the walls of the groove tend to push the balls out of the grove, and because of the spring-bias the balls can move in radial direction. Any error caused thereby will decrease the accuracy of the angular position-adjustment of the system. Similar angular position errors might take place when the balls cross the threading, because here the friction area between the grooves and the balls will change causing thereby an angular play.
A further problem connected with this design is that the bearings of the servo motor are not designed to resist higher axial forces. Furthermore, the ball screw drive is not able to provide a correct and efficient liner guide for the actuator shaft being the extension of the ball screw shaft, as the axial length of the ball screw is small. If such an actuator is coupled to any device which requires axial movement, a very accurate axial coupling or a separate coupling element is needed. Separate coupling elements (as described earlier) decrease the accuracy of angular position-control.
The actuator according to this prior document is not appropriate for being used for liquid dispensers, which require the same precision and accuracy within a wide range of different loads.
When a separate actuator is used for dispensing pumps just like in case of the aforementioned HIBAR 2S actuator, the design of the dispensing pump operated by the actuator can be freely chosen as long as it can be connected to the actuator. It is preferable if the actuator can be used for dispensing pumps working with liquids having a wide range of viscosity values, and wherein such dispensing pumps should be able to dispense precisely adjusted but different liquid volumes. When the output volume dispensed at a single stroke is increased, and especially if a liquid of higher viscosity should be dispensed, the movement of the plunger of the pump unit will require higher forces that present increased axial load that should be overcome by the force of the actuator. In case of higher axial loads, there will be proportionally high torques that the rotating ball screw shaft in the actuator unit should transmit to the rotor of the servo motor.
If such a type of actuator as described in the Kobayashi et. al publications is used for dispensing pumps allowing the use of a wide range of viscosities and dispensing volumes, then in case of higher loads a resilient slight twisting of the ball screw shaft and of the rotor will take place under the effect of the torque presented by the load. This is true, because the magnetic forces of the servo motor will act to the front portion of the rotor, and a step-wise narrowing rear part of the rotor is fixed to the rear end of the ball screw shaft. Under the effect of the torque the rotor will get slightly twisted, and the great length of the thin ball screw shaft between its rearmost attachment and the ball screw nut will also be twisted by a higher extent. It is customary, that a position encoder is attached to the rearmost portion of the rotor, and the position control of the servo motor is based on the angular position of this rearmost part of the rotor. Under the effect of the torque generated by the load of the pump, there will be a position error in the control owing to the angular displacement of the shaft and of the rotor. This will decrease precision of the dose-adjustment i.e. leads to a decreased accuracy when the pump requires higher forces. In other words, such actuators cannot be used for dispensing pumps with wide range of viscosities and/or dosage volume with the same degree of precision.
This means that prior art actuators of the aforementioned design have not only increased length but also had higher rotational inertias and a decreased precision, the degree of which was dependent on the load requirement of the attached dispensing pump.
When one wishes to decrease the overall size of the actuator system, a further problem will arise, namely how to dissipate the heat generated by the motor, especially when it is operated through long periods of time.